This will both render the total price on initial page load with Liquid, and keep the text of the <span> dynamically updated. Note the addition of the data-cart-view attribute, which tells Cart.js to dynamically update the contents of this element as the cart changes.

Implementing your var_net and var_tax dynamically will be a little more tricky. Cart.js does ship with math filters, but only plus and minus and they assume you're working only with integers (I just opened a ticket to fix that in the next release).

Fortunately, we can define our own formatters for Cart.js really simply! Try adding some code like this, after loading rivets-cart.min.js but before calling CartJS.init():